See the missing value for the 75th percentile of survival time for the group
exp==1? That is the question: why is that value
missing? In more extreme cases, you might see both the 50th and 75th
percentile estimates, or even all three, missing.

In any case, the short answer is that the estimates are missing because the
percentiles cannot be estimated. The percentiles of survival time reported
are for completed survival times, and they are obtained from the
Kaplan–Meier product-limit estimates of the survival curve.
Let’s look at that curve for this group:

. sts graph if exp==1

The survivor function S(t) reports the probability
of surviving at least to time t. The survivor
function here becomes flat at S(t) = .333. It does
that because not all the subjects have died yet—this is called right-censoring.

What is the 25th percentile of completed survival times? The 25th
percentile occurs where S(t) = 1−.25 =.75
(meaning that 25% have failed and 75% have yet to fail), and that is
t=1. Look back at the stsum
output and you will see that 1 is reported.

What is the 50th percentile of completed survival times? The 50th
percentile occurs where S(t) = 1−.50 = .50, and that is
t=2. stsum reports that number, too.

What is the 75th percentile? We do not know because more than 25% of the
subjects have not failed yet. Thus stsum reports
the 75th percentile for survival time as missing.